Two-way loudspeaker telephone installations



April 24, 1956 P. JEANLIN 2,743,314

TWO-WAY LOUDSPEAKER TELEPHONE INSTALLATIONS Filed Aug. 28, .1951 A s Sheets-Sheet 1 Fig.2

April 24, 1956 P. JEANLIN 2,743,314

TWO-WAY LOUDSPEAKER TELEPHONE INSTALLATIONS Filed Aug. 28, 1951 3 Sheets-Sheet 2 April 24, 1956 P. JEANLlN 2,743,314

TWO-WAY LOUDSPEAKER TELEPHONE INSTALLATIONS Filed Aug. 28, 1951 3 Sheets-Sheet 3 I DH ni e 5W8 Pa- LOUDSPEAKER TELEPHONE msrnnnn'nons Plerre-Jeanlin', Paris, France, assignor to Societe Le Teleampliphone, Paris, Erance,ra.corporation ohFrance f :Applieation August 28, 1951 Sl'ittlNO. 244,056

Cloimspriority, application France September 6,1950

' "4 Claims. or. 179-1 j lirwo-wnr microphone of the way inuse. Consequently the gain of'bothwayswas restrained when no speech current'was flowing-in one of the waysor channels. As soon as a modulation due to speech current appeared in one of the ways, the'gainof theway in use: was restored to its-full value, whereas the gain of the other way was completely nullified. But, due to this time constant (and-there is al ways a time constant even if small), the beginning of the speeclr' took place before the gainof the way in use was fully restored, so that the first syllables of the speech were often almost inaudible, thus introducing" a feature of great/practical nuisance; It is-ot course very ditlicult to understand a sentence the first. syllables of which are not clearly heard. And this defect is still greater when the while. speech consists of onlyone or two-syllables such as=yes no, of course, etc.

05 course the restrainingor blocking action is held as -lowas possible when no sound is being transmitted, but it must be nevertheless sufiicient to stop the loudest sound; able of reaching the loud spcak'er 'before the blocking of the other way. Should such a sound pass through the blocked amplifier the Larsen clfect'will start; But the restraining voltage being the same-when currents-corresponding to loud sounds or'wealt soundsflow through. the'way in use, a current corresponding to a weak will not energize the loud speaker before having tripped a= suflicient voltage for restoring the full gain of the restrained amplifier of the way in use" and nullifying the g ain of the amplifier-of the other way. It will be seen that the weaker sounds increase the timel'ag for restoring 'said'full gain and the clear hearing of said sound.

On the contrary, and according-to the present invention, the blocking action is not due to a source of voltage independent .of the modulated speech current, but: this speech current is by itself the source both of the-restraining or blocking. voltage andof the restoring;..vol'tage.

Consequently the same means. give rise vtotwo eltects': First,,it restrainsthe amplifier otsthe way in use according tothe intensity of the sound to be. transmitted. For a very weak soundthe restrainingvoltage is very low,

I so that said weak sound. can flow through without being completely restrained (a condition which isnot troublesome since such a weak sound. is unable to give rise to the f Larsen e'tfect). On the other hand; loud sounds are greatly restrained, but are nevertheless audible. Conse- 'quentl'y there is never total silence for the heater; Second,

'- it"promotes the restorationof the gain of the amplifier of the way in use and the blocking of the other way" at' the same time it promotes the restraint of the gain of 2,743,314 Patented the present invention has for its object the provision, irra two way loudspeaker installation each-way ot'which in cludes a microphone, a loudspeaker, and 'a-n= amplifier having an amplifying tube with an input circuit and an output'circuit, ofia restraining tube havinginputand output circuits respectively connected with the-outputand input circuits of the amplifying tube in such manner that the amplifying tube and restraining tube are notenen 'gizeti when no sound moduiated current-is flowing in either way and with the amplifying tube capable oF-e'xercising full gain, the restraining tubebeing-energiiedby the output circuit of the amplifyingtube'in response-to the appearance therein ofan amplified modulated current for supplying a gain restrictingpotential to the-input "circuit of the amplifyingtnbe. In additiomeach of' thfe ways is-pro vided' with a control circuit which is energized in responsev to sound modulated current appearing therein for supplying a cut-off potential to its restraining tube so that the gain restricting potential supplied by its restraining' tube is only temporaryand is applied onlyduring the initiation ot a conversation after which its-amplifying tube is restored to full gain, andfor supplying a blocking potential to the way not in use. I

According to the invention the auxiliary tubeis mounted in negative feed-back relation withthe amplifier tube with which it is combined. i This system makes use of'no element having an appreciable time constant, so that'the above mentionedjdrawback can no longer exist. r In a preferred mode ofcarryin the invention into pr actice, the'means used for blocking at the sarne'time 'thje negativefeed back tube combined with one-of-theampli fier tubes of the way or line in use and one of' theamplifier tubes of the way or line not'in use, is constituted,

in a-mannerin itself known", byan electronictubethc output of which leads to arectifi'er which gives a. continuous negative potential output utilized for the-two blockages above referred to. I

The accompanying drawings'illustrate diagrammaticab ing way or line.

In Figure 1, there are two loudspeaker telephone-stations PI and P2. The station PI comprises a microphone 100 and a loudspeaker 204, whilst the station Pz comprises amicrophone 200 and a loudspeaker 104. LI and 'L2- are the lines connecting the loudspeaker of each station" with the microphone of the other station.

In the'line L1 there is an amplifier A formed by three electronic tubes 101, 02,103. In the line L2 thereis-an amplifier B formed by three tubes 201, 202, and-203:;

I With the tubes 103- and 203 are combincdrespeetivdy auxiliary tubes. 105,, 205, the input andoutputvof these tubes 1105', 205, being coupled with the line Ll or 1.2 relatively to the tubes 103, 203, in suchmannerthat the voltage supplied by 1.05 or 205 is in phase-opposition .to the voltage arriving at 103 or 203.. There results when ,front, of 200.

is very high. At a point M in the line L1 there is connected a tube 106. The output of 106 is led to a rectifier 107 which then furnishes a continuous negative voltage. This voltage is fed to the grids of the tubes 105 and 201 which are thus blocked as soon as modulated current is emitted by the tube 102.

A symmetrical arrangement is constituted by the tube 206 and rectifier 207, which, when they are fed by the output from 202, block the tubes 205 and 101.

It will be seen at once that when current is established in the installation without any modulation being furnished by one of the microphones, the tubes 105 or 205 are not energized in feed back relation with the tubes 103, 203 so that the gain of said last-named tubes remains full but as soon as a speech current flows in one of the lines the tubes 105 or 205 act in negative feed-back relation with the tubes 103 or 203 insuch manner that only a weakened current nevertheless in proportion to the loudness of the sound emitted in front of the microphone in 1 use passes in the lines L1, L2. At the same time, with speech in front of one of the microphones, however, 100 for example, the modulated current traverses the tube 106 and passes to the rectifier 107 which, by the wires and 11, sends a negative voltage to the tubes 105 and 201.

This voltage is effectual for blocking the tube 105 in such manner that the negative feed-back to 103 ceases and the latter recovers its normal high gain. At the same time, however, the tube 201 is blocked through the wire 11. Consequently, the result is to restore full power to the line L1 and to block the line L2.

The inverse effect is produced if there is speech in p The current of the tube 206,-rectified at 207, blocks 205 and 101, thus restoring full power to 203 and blocking the line L1.

. It will be seen that the amplifier of the line used is only restrained, at the commencement of each conversation, in proportion with the intensity of the sound to be transmitted and during precisely the time necessary for the blocking of the amplifier of the other line. This action is practically instantaneous, the time constants of the system being practically nil as explained. hereinbefore.

The restraint, not being done by the direct action upon the amplifier tube of a continuous voltage from a rectifiedmodulation, introduces no risk of distortion. On the contrary, it atfords to the transmission the advantages derived from negative feed-back. Figures 2, 3 and 4 illustrate examples of means for carrying the invention into practice and they differ from one another mainly only in the points of connection of, the grids of the tubes 105 and 205. It should be noted that. the negative feed-back voltage should be 180- out of phase relative to the voltage of the corresponding tube 103 or 203. However, as the tubes 105, 205 themselves give an inversion of 180 it is necessary, in the event, to,

develop the grid voltage of the tubes 105 and 205 in such manner that, when developed, it will be of the same phase as the voltage of the tubes 103 and 203.

, In Figure 2, the grid voltage of the tube 105 is developed at the secondary of a transformer 1035 which; 4 serves for the coupling of the loudspeaker 104. It will be seen that the development is effected upon a potentiometer 1036 by way of a condenser 1037. The resist- -ance of the potentiometer. 1036 is in parallel with a resistance 1038 to which is connected the wire 10 coming from.

the rectifier 107 (Figure l).

Thefeed-back of the voltage emitted from 105 at 1051 is effected upon the grid 1056 of 103 and upon the anode 1011 of 101, and the grid 1057 of 101 is connected to the input ofthe tube 106 (Figure 1).

In Figure 3, the development of the grid voltage of 105 is as described with reference to Figure 2. The feedha'ck effected upon the grid 1032 of the tube 103 by decoupling the anode 1051 of the tube 105 by a resistance I ,1054 and a condenser 1055.

1 102 by connecting the cathode 1053 of the tube 105 with the cathode 1023 of the tube 102.

In all these examples, the negative feed-back voltage is adjusted by the potentiometer 1036, the cursor of which is connected to the grid 1052, under control of the condenser 1037. The resistance 1038 decoupled by the condenser 1039 completes the return from'the grid of 105 to the rectifier 107 which furnishes the blocking volt age.

The auxiliary amplifier 106 and the rectifier 107, which are of standard type, are not shown in Figures 2 to 5.

Figure 6 illustrates a mode of carrying out the invention as applied to a twowire-line loudspeaker telephone. This Figure is similar to Figure 1 and the operation is the same. However, the elements at the station P2 (microphone 200 and loudspeaker 104) are replacedby a differential transformer 301 of any known type as used for example in telephonic repeaters. The function of this ditferential transformer is to ensure the passage of the circuit from the four-wire circuit diagrammatically represented in Figure l to the two-wire circuit generally used in current telephony. An attenuator 302 can be provided for adjusting the level of emission and an equilibrator 303 can be added according to the type of differential circuit adopted.

It is to be noted that by reason of the qualities belonging to the circuits hereinbefore described, the equilibrium of the differential may be imperfect without altering the functioning of the system.

Figure 7 shows a detail of an arrangement of the auxiliary tube 105 adapted to the system in Figure 6. In Figure 7, the tube 403 replaces one of the tubes 103 or 203 in Figure 6.

The negative feed-back voltage of 105 is developed upon the cathode 4034 of 403 at the terminal of the biassing resistance 1033 and is applied to the grid 1052 of the tube 105 by way of the condenser 1037 and the dosing potentiometer 1036.

Blocking of the tube 105 by the current from the rectifier 107 is ensured by the return from the grid 1052 through the resistance 1036 decoupled by the condenser 1039.

The negative feed-back voltage, dephased by the tube 105, is picked up at 1059 (at the exit from the anode 1051 of 105) connected with the resistance 1055, through which the anode 1051 is fed by the high tension supply, marked +HT. The high tension voltage is applied to the grid 404 of the tube 403 by way of the condenser 410.

former-1061 with the differential transformer 301. The transformer 1061 may itself function as the differential transformer.

Although this arrangement has been described more particularly in regard to the system in Figure 6 in which the receiver is a differential transformer, it is equally adapted for being utilized with the system in Figure l in which the receiver is a loudspeaker.

In Figures 2 3, 4, 5, and 7 there is represented in the line L1 only one of the amplifier tubes 101 or 102, the arrangement of the second of these tubes in relation to the tube illustrated being apparent to all those acquainted with the art. Moreover, it is evident that the arrangements illustrated for the line L1 are equally suitable without change for the line L2.

I claim:

1. A two-way loudspeaker installation having a pair of transmission lines each of which comprises a microphone, a receiver, and an amplifier for transmitting signals from said microphone to said receiver, each of said amplifiers comprising amplifying means including an amplifying tube having input and output circuits, gain restraining means including a first control tube connected with said output circuit and temporarily responsive to the transmission of signals through said output circuit for supplying a gain reducing potential to said amplifying tube, and means including a second control tube connected with said input circuit and responsive to the transmission of signals from said microphone for supplying a cut-off potential to said first control tube and to the amplifying tube of the amplifying means in the other of said lines.

2. An installation as claimed in claim 1 characterized by said last-named means including arectifier energized by the output of said second control tube, and having an output circuit connected with said first control tube and with said amplifying tube in the other of said lines.

3. A two-way loudspeaker installation having a pair of transmission lines each of which comprises a microphone, a receiver, and an amplifier for transmitting signals from said microphone to said receiver, each of said amplifiers comprising amplifying means including an amplifying tube having input and output circuits, gain restraining means including a first control tube connected with said output circuit and temporarily responsive to the transmission of signals through said output circuit for supplying a gain reducing potential to said amplifying tube, means including a second control tube connected with said input circuit and responsive to the transmission of signals from said microphone for supplying a cut-01f potential to said first control tube, and means connecting each of said last named means in one of said lines with the amplifying means in the other of said lines whereby to render the amplifying means in either line inoperative in response to transmission of signals by the amplifying means of the other line.

4. A two-way loudspeaker installation having a pair of transmission lines each of which comprises a microphone, a receiver, and an amplifier for transmitting signals from said microphone to said receiver, each of said amplifiers comprising amplifying means including an amplifying tube having input and output circuits, gain restraining means including a first control tube connected with said input circuit and control means connected with said output circuit responsive to the transmission of modulated current through said output circuit for rendering said control tube operative to supply a gain reducing potential to said input circuit, and meansincluding a second control tube connected with said input circuit and responsive to the transmission of signals from said microphone for supplying a cut-off potential to said first control tube and to the amplifying tube of the amplifying means in the other of said lines.

References Cited in the file of this patent UNITED STATES PATENTS 2,236,645 Levy Apr. 1, 1941 2,545,466 Jeanlin Mar. 20, 1951 2,545,467 Jeanlin Mar. 20, 1951 2,545,476 Levy Mar. 20, 1951 2,545,477 Levy Mar. 20, 1951 

